In processing a wafer to fabricate a semiconductor device, it is important to prevent particles from contacting the wafer surface. Because even fine particles may greatly affect formation and operation of semiconductor devices, maintaining a clean environment and performing cleaning at each wafer processing step can be very important.
A wafer is typically cleaned by a wet cleaning process using a cleaning solution. Even though the cleaning solution may be varied with process characteristics, pure water (i.e., deionized water) is conventionally used to clean chemical materials or particles from a wafer.
There is a need for drying the cleaned wafer without affecting the surface of the cleaned wafer. If the cleaned wafer is exposed to water for a long time, a substrate surface material of the wafer may be denaturalized. Even weak denaturalization may cause substantial problems for large-scaled semiconductor devices. A spin drying method may be used as a wafer drying method. If the spin drying method is used, liquid is easily removed but particles are likely to reattach to the wafer. Alternatively, the Marangoni wafer drying method may be used. The Marangoni wafer drying method uses the surface tension of a liquid. The Marangoni wafer drying method will be described hereinbelow with reference to FIG. 1 and FIG. 2.
Referring now to FIG. 1, a wafer 60 is submerged in deionized water 40 in a liquid bath 20 in a dry chamber 10. Organic liquid vapor such as isopropyl alcohol (IPA) vapor 70 is provided to and around a surface of the deionized water 40 through a vapor pipe 50. Referring now to FIG. 2, the organic liquid vapor is continuously provided, and the submerged wafer 60 is removed from (i.e., exposed to the exterior of) the deionized water 40 by draining the deionized water 40 through a drain valve 85 of a drain pipe 80. Alternatively, the wafer 60 may be carried through a surface of deionized water.
During the foregoing process, the organic liquid vapor is condensed or dissolved to form a liquid film such as an IPA film 30. A differential between the surface tensions of the IPA film 30 and the deionized water 40 at an interface therebetween serves to prevent particles from reattaching to a surface of the wafer 60 crossing the interface and also serves as a force to remove liquid from the surface of the wafer 60. Diacetone and 1-methoxy-2-propane as well as IPA may be used as the organic liquid.
When the wafer is submerged in the deionized water and the organic liquid vapor is first provided, a vapor supply volume or a vapor supply time must be sufficient to raise the concentration of the organic liquid in the dry chamber 10 to a predetermined level. Preferably, as the wafer is lifted from the deionized water or the surface of the water is lowered to dry the wafer, the volumetric supply rate of the vapor (i.e., the volume of the supplied organic liquid vapor supplied per unit of time (for example, liters per minute)) is reduced to a predetermined level in order to stabilize the organic liquid concentration at the surface of the deionized water as well as the partial pressure of the organic liquid vapor in the dry chamber 10. If the partial pressure of the organic liquid vapor is high as the deionized water is slowly drained, the organic liquid may condense on the wafer surface at locations not submerged in the water. The condensed organic liquid may be removed in a subsequent drying step, which may increase the concentration of particles on the wafer at such locations.
FIG. 3 is a graph illustrating the above-mentioned problems associated with particles, wherein “pre” refers to the number of particles per area on a wafer tested before the wafer is dried, “after” refers to the number of particles measured after the wafer is dried by the conventional Marangoni drying method using IPA vapor, and “diff” refers to a difference in the number of particles measured before and after the wafer is dried.
FIG. 4 illustrates a conventional wafer drying apparatus that supplies organic liquid vapor using a bubbling method. In such a wafer drying apparatus of the Marangoni type, the organic liquid vapor is restrictively supplied by a pressurized bubbling gas source through one bubbling gas supply pipe 120. Therefore, if the bubbling gas source pressure is high so that the volumetric supply rate of the supplied vapor is large, the time required for a first step of drying the wafer may be reduced. However, the concentration of organic liquid on the interface surface of the deionized water may become high as a result of organic liquid vapor supplied in a second step. Thus, particles may increase in number, presenting a high likelihood that particles will reattach to the wafer.
If the bubbling gas pressure is low, the time required to supply sufficient organic liquid vapor to raise the concentration of the organic liquid on the surface of the deionized water to the predetermined level becomes long. This may result in a long process time, as well as erosion and denaturalization of a metal layer or the like formed on a semiconductor substrate.